The particular purpose of optoelectronic sensor devices is to detect the quantity and/or quality in a representative form of the moisture precipitating per unit of time on the front or rear windscreen of a motor vehicle and in dependence thereupon, to influence automatically a windscreen wiping system allocated to the screen.
Devices for controlling a motor-driven windscreen wiping device have become known from DE 32 43 373 A1, DE 33 14 770 C2, DE 40 06 174 C1, DE 40 06 420 A1, DE 40 17 063 A1 and DE 40 19 066 A1, wherein by way of a beam guide element attached to the inner surface of a transparent windscreen, beams emitted from an associated beam transmitter are coupled into the transparent windscreen and after at least one reflection at the outer surface of the windscreen are decoupled again by way of the beam guide element and are directed to an associated beam receiver.
Some multi-layer windscreens installed nowadays in motor vehicles have an integrated intermediate metallic layer which is extremely thin and thus hardly impairs the transparency of the windscreen. This said intermediate layer is provided on the one hand to provide possible electrical heating of the windscreen and on the other hand to provide thermal insulation against infra-red rays which effect the motor vehicle from the outer side. This type of windscreen leads to the problem that the beams emitted by the beam transmitter are likewise reflected to a considerable extent at this intermediate layer and they are thus not available for the measuring process. Moreover, these beams stray as spurious beams in the beam guide element and they can thus have a negative influence on the signal supplied by the beam receiver. It has also been established that the spurious beams change in dependence upon the wavelengths of the beams emitted by the beam transmitter, these wave-lengths varying with the different temperatures.